Positioning systems are used in a wide variety of fields, wherein the basic principles of the systems are normally the same. In general, a positioning system is used to move one or several elements to a respective desired location in a working space, often such that a work head is positioned at a desired location in relation to a target space. The elements are usually work heads and/or target spaces, which are positioned at a desired location entered into the machine, by the use of positioning means controlled by a control unit.
Herein, the term working space refers to a space comprising the coordinates at which the element(s) can be positioned by the positioning system. Further, if items or material can be placed at predetermined locations on e.g. the target space, by for instance a mounting head or dispensing head, these locations are normally also comprised in the working space.
There are many alternative ways of arranging the control of the positioning of a work head in relation to a target space. In some systems, the work head is fixed, and the target space is moved by the positioning system such that the work head is positioned correctly in relation to the target space. In other systems the target space is fixed, and the work head is moved by the positioning system. In yet other systems, both the work head and the target space are moved by the positioning system. In for example a split axis machine the work head is moved along a first axis, and the target space is moved along a second substantially orthogonal axis.
The working space, or at least a portion of the working space, is normally spanned by two different sets of coordinate systems, one actual or real coordinate system and one nominal or machine coordinate system. Additionally, there is a set of conversion values for transforming a coordinate expressed in one of the coordinate systems, into a corresponding coordinate expressed in the other coordinate system. This is sometimes referred to as mapping.
Herein, the term actual coordinate system refers to a coordinate system which is independent of the positioning system, generally orthonormal, and normally the coordinate system which the user is most familiar with. Typically, CAD data related to the target space is first described in the actual coordinate system and later converted into the nominal coordinate system.
The term nominal coordinate system refers to a coordinate system which is determined by the structure and properties of the positioning system. The nominal coordinate system is in most cases not perfectly orthonormal due to imperfections in the positioning system, such as misalignment of a positioning axis, or roll, pitch and yaw associated with the positioning of the work head.
The term conversion values refers to a set of numerical values, which are used for converting or transforming an actual coordinate into a nominal coordinate, or vice versa. Usually, one aim when calibrating a positioning system is to determine the conversion values, such that when they are applied to an actual coordinate, indicating a first location in said working space, the result is a nominal coordinate indicating the exact same location. Such a pair of actual and nominal coordinates are referred to as a corresponding pair of coordinates. Hence, if correct values are entered into the machine and if the positioning system is perfectly calibrated, i.e. the conversion values are ideal, then there will be no difference between the desired position of the element and its actual location after it has been positioned by the positioning system.
A conventional method of calibrating a three-dimensional positioning system, wherein the target space is two dimensional, e.g. a circuit board, is to use a calibration board which essentially covers the target surface of the positioning system. The calibration board is provided with reference markings and the location of each reference marking on the calibration board is known with better accuracy than the required accuracy of the positioning system. The locations of the reference markings are measured by one or more sensors, which scan the calibration board under the control of the positioning system. The sensed location of each reference marking in nominal coordinates is compared to their respective known locations in actual coordinates, and a corresponding correction value is computed for each reference marking on the reference board.
After the calibration, nominal coordinates within a portion of said working space will be calibrated. A selected working space refers herein to a portion of the working space which is to be calibrated. A selected portion of the working space might comprise the whole of said working space.
There are problems related to the above described calibration procedure used in prior art. First of all, there is a practical concern. Different machines have target spaces of different sizes. In order to be able to calibrate the whole target space for all types of machines, the operator conventionally uses a set of different calibration boards, one for each size of target space. Alternatively, a calibration board which fits the smallest target space can be used in all machines, but this may leave portions of the target space uncalibrated.
Another concern is the cost for manufacturing large calibration boards which provides for a sufficiently high accuracy. Particularly as these boards should withstand at least moderate variations in temperature and also be durable over time. This puts high demands on the materials as well as the manufacturing processes used. If the quality of the calibration is not sufficiently high, due to for example a poor calibration board, the accuracy of the positioning system will be affected accordingly.
The accuracy in the positioning of the work head is crucial in many applications, e.g. in machines used in relation to circuit board manufacturing, and specifically to the steps required for circuit board assembly.
There are several different types of machines which are used in relation to circuit board assembly, e.g. dispensing, mounting and/or inspection machines.
In a component mounting machine, the work head or component placement head is used to pick a desired electric component from a supply of components, and place the component at a desired location on the circuit board. It is important that the components are correctly attached to the circuit board, and even more important that they are placed with high accuracy. Otherwise, the risk of a subsequent malfunction of the board is impending.
In a dispensing machine, a work head or dispensing head holds a dispenser which distributes a viscous medium, e.g. solder paste, glue or underfill, with high accuracy in desired positions on a circuit board, in order to attach electric components to the board. The arrangement of said viscous medium is critical, as mistakes often cause an unsatisfactory connection between the component and the board.
There are several types of automatic inspection machines, e.g. one which uses X-rays and another which uses visible light. In both of these systems, a work head or optical inspection head is used to position a detector, which is sensitive to the electromagnetic radiation in question, such that the attachment, location and orientation of the mounted components and/or the applied viscous medium can be verified. Other types of inspection machines use touch probes for contact inspection of objects. Then, the touch probe is mounted on a work head controlled by a positioning system.
The term self calibration refers to the use of an imperfectly calibrated measuring instrument and one or more imperfectly calibrated reference device(s) to improve the calibration of the instrument and the reference device(s), wherein said measuring instrument is used by e.g. a positioning system.
When a reference device is imperfectly calibrated, the relative positions between the reference features are not known with a sufficient accuracy, an accuracy that e.g. is substantially higher than the accuracy one wants to achieve for the positioning system after calibration. The reason that the reference device is imperfectly calibrated may be that the reference device has never been measured with such an sufficient accuracy or changes in the reference device due to wear or thermal variations.
There is an ever increasing demand for smaller circuitry based consumer products. In order to meet this demand, the accuracy of the positioning systems must normally be improved accordingly, such that smaller tolerances can be achieved. It is also desirable that the idle time of the machine is as short as possible.